The difference in time delay of δ18O, δ13CDIC and Mg/Ca to the hydroclimate change monitored in a subtropical cave, South China

Abstract

Speleothem δ¹⁸O, δ¹³C_DIC, and Mg/Ca are widely used to reconstruct past climatic and environmental changes. However, due to the rainfall regime and hydrological structure of caves, their application in reconstructing past hydroclimate in the East Asian monsoon region is poorly understood. In this study, we monitored two types of drip water in a subtropical cave to investigate the different responses of δ¹⁸O, δ¹³C_DIC, and Mg/Ca to hydroclimate changes at monthly to yearly scales. Our results showed that drip rate at all drip sites responded quickly to precipitation, whereas δ¹⁸O, δ¹³C_DIC, and Mg/Ca responded with some lag. The lag time for the parameters were in the order of δ¹³C_DIC (lag time ranging from 1 to 2 months) < Mg/Ca ratio (lag time of 2 months) < δ¹⁸O (lag time up to 5 months), and for fissure-fed drip sites (M-5 and M-6) < seepage flow drips (M-1 and M-2), as well as single-reservoir supply (M-5) < multi-reservoirs supply (M-6). A comprehension of both fast and slow flows is crucial in order to comprehend karst reservoirs and their paths, which in turn enables the exploration of the hydrological properties of unsaturated zones under the context of Asian monsoon. We also found that the hydrological structure was crucial in generating the different response times of δ¹⁸O, δ¹³C_DIC, and Mg/Ca in drip water. Additionally, the relationship between δ¹⁸O and δ¹³C_DIC may be due to the different residence times in different drip sites on seasonal- to annual timescales.